Successful installation of second volumes in Mac mini late 2012
The reason to follow this guide was to install a SSD as boot or main volume
The guide was was easy to follow. I also found the comments very helpful and installed the original HD in the upper bay with the ribbon cable from iFixit and the SSD with the original ribbon cable in the lower bay.
Follow the guide exactly as written with the exception of the SSD installation bay. Here it is safer to follow the comments.
I use some additional tools which proved to be helpful:
(1) A pair of cross locking tweezers to grab screws
(2) An organizer with 9 compartments to hold the screws, sorted by step number; additionally I used some pieces of paper with the step number which went also in the compartment